Waveguides assembled for transverse-transfer of optical power

ABSTRACT

Formation of a substantially flat upper cladding surface over a waveguide core facilitates transverse-coupling between assembled waveguides, and/or provides mechanical alignment and/or support. An embedding medium may be employed for securing optical assemblies and protecting optical surfaces thereof. Structural elements fabricated with a low-profile core may be employed for providing mechanical alignment and/or support, aiding in the encapsulation process, and so forth.

RELATED APPLICATIONS

[0001] This application claims benefit of the following U.S. provisionalapplications:

[0002] Application Ser. No. 60/393,974 entitled “Micro-hermeticpackaging of optical devices” filed Jul. 5, 2002 in the names of AlbertM. Benzoni, Henry A. Blauvelt, David W. Vernooy, and Joel S. Paslaski,said provisional application being hereby incorporated by reference asif fully set forth herein; and

[0003] Application Ser. No. 60/466,799 entitled “Low-profile-core andthin-core optical waveguides and methods of fabrication and use thereof”filed Apr. 29, 2003 in the names of David W. Vernooy, Joel S. Paslaski,and Guido Hunziker, said provisional application being herebyincorporated by reference as if fully set forth herein.

BACKGROUND

[0004] The field of the present invention relates to optical waveguides.In particular, various adaptations are disclosed herein for facilitatingassembly of planar optical waveguides for transverse-transfer of opticalpower therebetween.

[0005] This application is related to subject matter disclosed in:

[0006] U.S. non-provisional App. No. 10/187,030 entitled “Opticaljunction apparatus and methods employing optical powertransverse-transfer” filed Jun. 28, 2002 in the names of Henry A.Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S. Paslaski, saidapplication being hereby incorporated by reference as if fully set forthherein;

[0007] U.S. provisional App. No. 60/360,261 entitled“Alignment-insensitive optical junction apparatus and methods employingadiabatic optical power transfer” filed Feb. 27, 2002 in the names ofHenry A. Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S.Paslaski; and

[0008] U.S. provisional App. No. 60/334,705 entitled “Integratedend-coupled transverse-optical-coupling apparatus and methods” filedOct. 30, 2001 in the names of Henry A. Blauvelt, Kerry J. Vahala, PeterC. Sercel, Oskar J. Painter, and Guido Hunziker.

SUMMARY

[0009] A first planar optical waveguide includes a first core withinlower-index first cladding on a first waveguide substrate. Asubstantially flat waveguide upper cladding surface is provided over thefirst core along at least a portion of the length thereof. A secondplanar optical waveguide includes a second core within lower-indexsecond cladding on a second waveguide substrate. A substantially flatwaveguide upper cladding surface is provided over the second core alongat least a portion of the length thereof. The first and second planarwaveguides are assembled together with at least portions of theircorresponding substantially flat waveguide upper cladding surfacesfacing one another, thereby positioning the waveguides fortransverse-transfer of optical power therebetween along respectivetransverse-coupled portions of the first and second cores. The waveguideupper cladding surfaces may be positioned against one another uponassembly of the waveguides, or may be spaced apart from one another.

[0010] Additional areas of core material may be provided within thecladding so as to provide corresponding structural upper claddingsurfaces in a manner similar to that employed for forming the waveguideupper cladding surfaces. The waveguide and structural upper claddingsurfaces are substantially parallel, and may in some instances also besubstantially coplanar. The structural upper cladding surfaces arepositioned against one another upon assembly of the planar waveguides,thereby providing alignment and/or support.

[0011] Substantially flat waveguide and structural upper claddingsurfaces may be formed by deposition of cladding material over alow-profile core, wherein the width of the core (i.e., the lateraldimension) is larger than the height of the core (i.e., the verticaldimension). Alternatively, substantially flat waveguide and structuralupper cladding surfaces may be formed by chemical-mechanical polishing(CMP) or other suitable processing of cladding deposited over a core ofany shape.

[0012] An embedding or encapsulating medium may be employed for securingthe assembled planar waveguides and protecting various optical surfacesthereof. Structural upper cladding surfaces formed by areas of corematerial may be employed for aiding in the encapsulation process, byproviding mechanical alignment and/or support during the embeddingprocess, and/or by directing flow of embedding material precursor(s) tothe appropriate locations about the waveguides. Such embedding may serveas a micro-hermetic package and/or may serve to enhance opticalproperties/performance of the packaged assembled optical waveguides.

[0013] Objects and advantages pertaining to optical waveguides assembledfor transverse-coupling as disclosed herein may become apparent uponreferring to the disclosed exemplary embodiments as illustrated in thedrawings and disclosed in the following written description and/orclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIGS. 1A-1E are cross-sectional views of exemplarylow-profile-core optical waveguides.

[0015] FIGS. 2A-2E are cross-sectional views of exemplarylow-profile-core optical waveguides.

[0016] FIGS. 3A-3C are cross-sectional views of optical waveguides.

[0017] FIGS. 4A-4D are plan and cross-sectional views of assembledoptical waveguides.

[0018] FIGS. 5A-5D are cross-sectional and plan views of assembledlow-profile-core optical waveguides.

[0019] FIGS. 6-11 illustrate exemplary procedures for forming waveguideand structural upper cladding surfaces.

[0020] FIGS. 12A-12B are longitudinal and transverse cross-sectionalviews of assembled and embedded low-profile-core optical waveguides.

[0021]FIG. 13 is a plan view of a low-profile-core optical waveguide.

[0022]FIG. 14 is a plan view of optical devices assembled onto awaveguide substrate.

[0023] The embodiments shown in the Figures are exemplary, and shouldnot be construed as limiting the scope of the present disclosure and/orappended claims. It should be noted that the relative sizes and/orproportions of structures shown in the Figures may in some instances bedistorted to facilitate illustration of the disclosed embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

[0024] Low-profile cores as shown in the exemplary waveguides of FIGS.1A-1E and 2A-2E may offer advantages for fabrication and assembly oftransverse-coupled optical waveguides. Once the core material layer fora waveguide is deposited and then patterned to form the core, additionalcladding material is typically deposited to continue the fabricationprocess. Deposition processes typically employed exhibit varying degreesof conformality, and when cladding material is deposited over awaveguide core so that the upper cladding thickness and core height aresimilar (within about a factor of two, for example), a curved claddingupper surface typically results. For a waveguide core with an aspectratio (width:height) of less than about 2:1, a majority of the uppercladding surface over the waveguide is typically curved (waveguide 1250,core 1252, and cladding 1254 on substrate 1256, as shown in FIG. 3A). Incontrast, under similar circumstances a low-profile core (having anaspect ratio greater than about 2:1 (waveguide 1260, core 1262, cladding1264, and substrate 1266, as shown in FIG. 3B; other examples shown inFIGS. 1B and 2B) may typically yield an upper cladding surface curvednear the lateral edges of the core but substantially flat above amajority (if not all) of the width of the core. The resultingsubstantially flat waveguide upper cladding surface may facilitateassembly of the waveguide with another similarly fabricated waveguidewith their waveguide upper cladding surfaces facing one another (eitheragainst one another, as in FIG. 3C, or spaced-apart from one another).The substantially flat waveguide upper cladding surfaces facilitatestable and reproducible positioning of the waveguides fortransverse-transfer of optical power.

[0025] The substantially uniform thickness of material layer(s)deposited to form waveguide core 1262, and substantially uniformdeposition processes available for forming cladding 1264 (and the uppersurface thereof) provide several advantages. Over short length scales (afew millimeters or less) the portion of the upper cladding surfacedirectly over the waveguide core 1262 (a waveguide upper claddingsurface) is substantially flat and uniform, thereby enabling anotherwaveguide 1270 to be positioned on waveguide 1260 against the uppercladding surface and providing reproducible and stable relativepositioning of the two waveguide cores, with negligible interveningspace between the waveguide upper cladding surfaces (FIG. 3C).Substantial uniformity of deposition over larger length scales (a fewcentimeters or more) yields multiple substantially identical waveguides1260 (including substantially identical upper cladding layers) formedconcurrently on a common substrate wafer (wafer-scale fabrication). Theoptical performance of substantially identical waveguides reproduciblyand stably positioned for optical transverse-coupling may therefore berelied on for reliable fabrication and assembly of optical devices.

[0026] Transverse-coupled portions of waveguides 1260 and 1270 of FIG.3C (as well as transverse-coupled portions of other exemplary waveguidesdescribed and/or shown herein) may be adapted for substantiallymodal-index-matched transverse coupling, or may be adapted forsubstantially adiabatic transverse-coupling. Such adaptations fortransverse-coupling are set forth in detail in earlier-cited applicationSer. No. 10/187,030, application Ser. No. 60/360,261, and applicationSer. No. 60/334,705. One such adaptation for substantially adiabaticcoupling is shown in FIGS. 4A-4D, which show a core 1112 of a firstwaveguide tapering down in width until it terminates, while a core 1122of a second waveguide, transverse-coupled with the first waveguide,originates at a point below core 1112 and tapers up in width as core1112 tapers down.

[0027] FIGS. 1A-1E show cross sections of exemplary embodiments of aplanar optical waveguide including a low-profile core 310. The waveguideis formed on a waveguide substrate 302, typically a substantially planarsemiconductor substrate such as silicon. Any suitable waveguidesubstrate material may be employed, including those listed hereinbelowand equivalents thereof. Core 310 is surrounded by lower-index cladding320. In the examples of FIGS. 1A-1E, the waveguide core 310 may comprisea thin layer of silicon nitride (Si_(x)N_(y); index of about 2) orsilicon oxynitride (SiN_(x)O_(y)), typically ranging between a few tensand a few hundreds of nanometers in thickness (i.e., vertical extent).Cladding 320 in this example may comprise silica or doped silica (indexaround 1.5), so that these exemplary waveguides are high-index-contrast(defined herein as a core/cladding index contrast greater than about5%). Other suitable high-index-contrast combinations of core andcladding materials may be equivalently employed, including those listedhereinbelow and equivalents thereof.

[0028] For supporting optical modes at typical telecommunicationswavelengths (visible and near-infrared), core 310 of the exemplaryhigh-index-contrast waveguide 300 may range between about 0.5 μm andabout 8 μm in width (i.e., lateral extent). The particular vertical andlateral extents chosen depend on the desired characteristics ofwaveguide 300 (described further hereinbelow). A silicon nitride corearound 50-100 nm high by about 2-3 μm wide (yielding a transverse modesize of around 1-2 μm high by around 1.5-2 μm wide; mode sizes expressedas 1/e² HW intensity) might be well-suited for spatial-mode-matchingwith a semiconductor optical device mode, while an even thinner (invertical extent) silicon nitride core around 5-6 μm wide may bewell-suited for spatial-mode-matching with larger optical modes, or forfacilitating optical transverse-coupling with another waveguide.

[0029] The cladding 320 between substrate 302 and core 310 may be madesufficiently thick so as to reduce or substantially prevent leakage ofoptical power from waveguide 300 into substrate 302 (withinoperationally acceptable limits). A lower cladding thickness greaterthan about 5 μm, typically greater than about 10 μm, may adequatelyserve this purpose. In embodiments formed on a silicon or othersemiconductor substrate, an oxide buffer layer is sometimes provided onthe substrate. Such a buffer layer may comprise the lower cladding, ormay comprise an upper surface of the substrate on which the lowercladding is deposited. Other suitable structures may be employed forsubstantially preventing optical leakage from the waveguide into thesubstrate while remaining within the scope of the present disclosureand/or claims.

[0030] The thickness of cladding 320 above core 310 may vary, dependingon the intended use of the waveguide. Along portions of the length ofthe waveguide, the upper cladding may be made sufficiently thick so asto reduce or substantially prevent optical leakage through the uppersurface of the waveguide (within operationally acceptable limits),and/or to substantially isolate a supported optical mode from a useenvironment (within operationally acceptable limits). An upper claddingthickness greater than about 5 μm, typically greater than about 10 μm,may adequately serve this purpose. Along a transverse-coupled portion ofthe length of the waveguide, a thinner upper cladding layer may be moresuitable. Such a thinner upper cladding may typically be less than about1 μm in vertical extent, and often less than about 0.5 μm in verticalextent, in order to facilitate transverse spatial overlap betweenoptical modes of transverse-coupled waveguides. In other examples, athin upper cladding layer may be adequate in cases where the waveguideis subsequently embedded or encapsulated in a transparent optical mediumhaving an index less than or about equal to the cladding index. Ineffect, the embedding medium acts as additional cladding.

[0031] Depending on the physical and/or mechanical constraints and/orrequirements imposed on the waveguide, the cladding 320 may extendlaterally away from the core 310 so as to reduce or substantiallyeliminate any effect of a lateral waveguide cladding edge on an opticalmode supported by core 310 (as in FIGS. 1B and 1E). Alternatively,cladding 320 may be formed so as to yield a protruding lateral surfaceon one or both sides of core 310 (as in FIGS. 1A, 1C, and 1D), and suchsurface(s) may or may not influence the characteristics of a supportedoptical mode. Such lateral cladding surfaces may be provided at varyingdepths, and may or may not extend downward near or beyond the depth ofcore 310. A waveguide may be formed to include multiple segments havingvarious of these configurations.

[0032] Additional exemplary embodiments of a planar waveguide includinga low-profile core are shown in cross-section in FIGS. 2A-2E positionedon a waveguide substrate 402. Substrate 402 may comprise a semiconductorsubstrate such as silicon (as in the preceding examples), or anysuitable substrate material may be employed, including those listedhereinbelow and equivalents thereof. In these examples the waveguide mayinclude a doped silica core 410 within lower-index cladding 420, whichmay comprise doped or undoped silica. The index contrast is typicallymuch smaller than in the examples of FIGS. 1A-1E, and may be less thanabout 1 or 2%, for example (low-index-contrast, defined herein ascore/cladding index contrast less than about 5%). The core may be about0.5 μm high by about 5 μm wide in this example, yielding a transversemode size of around 4-5 μm high by around 4-5 μm wide (mode sizesexpressed as 1/e² HW intensity). Such a mode might be well-suited forspatial-mode-matching with an optical fiber mode. A low-profile,low-index-contrast waveguide core may range from about 0.3 μm high up toabout 2-3 μm high, and between about 1 μm and about 10 μm wide. Specificcombinations of dimensions will depend on the desired spatial modecharacteristics and the particular degree of index contrast employed. Inaddition to doped and undoped silica, other suitable low-index-contrastcombinations of core and cladding materials may be equivalentlyemployed, including those listed hereinbelow and equivalents thereof.

[0033] As in the previous examples, cladding 420 below core 410 may besufficiently thick so as to reduce or substantially eliminate opticalleakage from waveguide 400 into substrate 402 (within operationallyacceptable limits), alone or in combination with a buffer layer providedon the substrate. Upper cladding may be sufficiently thick alongportions of the waveguide so as to substantially prevent optical leakagethrough the upper surface of the waveguide, and/or may be sufficientlythin along other portions of the length of the waveguide so as tofacilitate optical transverse-coupling between waveguides. Lateralportions of cladding 420 may be configured in any of the various waysdescribed hereinabove, and waveguide 400 may be formed to includemultiple segments having various of these configurations.

[0034] The structural properties of the layers and spatially selectivematerial processing steps used to form a waveguide with a low-profilecore may be further exploited for forming nearby structural members foralignment and/or support of optical waveguides assembled fortransverse-coupling. In FIGS. 5A, 5B, and 5D, a pair of elongated areas1330 of core material are shown on either side of core 1310, allpatterned from a common layer of core material deposited on a lowerlayer of cladding 1320 on substrate 1302. Additional areas 1340 may besimilarly patterned from the core material layer. Since core 1310 andstructural areas 1330 and 1340 are formed concurrently from the samecore material layer, their respective upper surfaces are substantiallycoplanar. Deposition of additional cladding 1320 yields substantiallyflat areas of the upper cladding surface of waveguide 1300 (above core1310, forming a waveguide upper cladding surface) and above areas 1330and 1340 (forming structural upper cladding surfaces). These respectiveupper cladding surfaces are substantially coplanar, and thus yield anenlarged mechanical mating surface for another similarly adaptedwaveguide 1350 (including core 1351, cladding 1352, and structural areas1353 and 1354 on substrate 1355 along with optical device 1356; shown inFIGS. 5A, 5C, and 5D) assembled therewith for opticaltransverse-coupling. The enlarged contact surface area between theassembled waveguides reduces the likelihood of mechanical damage to thewaveguides during positioning, alignment, and bonding (compression,thermal, soldering, or otherwise) and provides more stable andreproducible positioning relative to assembled waveguides lacking suchenlarged areas of mechanical contact. Patterning of the structuralmembers 1330/1340 from the same layer as the waveguide core 1310 (andsubsequent concurrent deposition of cladding material 1320 over all)results in structural surfaces that are well aligned with respect to therelevant optical surfaces. Similarly, patterning of structural members1353/1354 from the same layer as the waveguide core 1351 (and subsequentconcurrent deposition of cladding 1352 over all) similarly results instructural surfaces that are well aligned with respect to the relevantoptical surfaces.

[0035] The outlying structural areas 1340/1354 may be disposed about thewaveguide cores 1310/1351, respectively, so as to provide additionalalignment and/or support for the assembled substrates and to facilitatemanipulation and placement of substrate 1355 on substrate 1302 (forassembling together the waveguides for transverse-coupling). FIG. 5Dshows an outline of substrate 1355 (along with corresponding waveguidecore 1351 and alignment/support structural members 1353/1354) positionedover substrate 1302. An outline or “footprint” 1359 for a device used tohandle substrate 1355 is also shown. If the outlying alignment/supportstructural members 1340/1354 are positioned sufficiently far apart,non-parallelism of substrates 1302 and 1352 will not cause furthertilting of the substrates relative to one another (and possibleseparation of the substrates at one edge), but will rather result in thesubstrates being forced together into a substantially parallelarrangement with the respective structural upper cladding surfacespositioned against one another. The substrates 1302 and 1355 may befurther provided with metal solder contact areas and/or solder pads (notshown). These may be arranged so as to make first contact as thesubstrates are brought together for assembly. Heating the solder to itsreflow temperature enables the substrates to further settle toward oneanother, until the respective structural upper cladding surfaces makecontact. The solder is then allowed to cool and solidify with thesubstrates held in this fully engaged position.

[0036] Exemplary dimensions and positions that might be employed forforming waveguide core 1310 and alignment/support structural members1330 may be about 6 μm wide for core 1310 and structural members 1330(formed from a silicon nitride layer about 50-100 nm thick in thisexample), with about a 9 μm separation between the core and the adjacentstructural members. Corresponding exemplary dimensions for structures onsubstrate 1355 may be about 2 μm for waveguide core 1351 and about 10 μmwide for alignment/support structural members 1353 (formed from asilicon nitride layer about 50-100 nm thick in this example), with abouta 9 μm gap between the waveguide core and adjacent structural members.The greater width of waveguide core 1310 relative to waveguide core 1351yields a broader range of lateral positions over which an operationallyacceptable level of optical transverse-coupling may be achieved, whilethe greater widths of structural members 1353 relative to structuralmembers 1330 yields a correspondingly larger range of lateral positionsover which adequate mechanical engagement is maintained. In particular,there may be instances wherein operationally acceptable opticaltransverse-coupling might be achieved with negligible (or only oblique)mechanical contact between waveguide upper cladding surfaces. In suchinstances, contact between the structural upper cladding surfacesprovides the needed mechanical alignment and support, despite asubstantial lack thereof near the waveguide cores. Many suitable sets ofnumber, shapes, positions, and/or dimensions for various alignmentand/or support structural members may be employed (in addition toexemplary configurations set forth herein), depending on the opticaland/or mechanical characteristics desired for the assembledtransverse-coupled waveguides.

[0037] The structural members 1330 should be far enough from core 1310to substantially avoid (within operationally acceptable limits) opticaltransverse-coupling therewith, and to similarly substantially avoidoptical transverse-coupling with waveguide core 1351 upon assembly (overa range of relative waveguide positions within the assembly tolerance).Analogously, structural members 1353 should be far enough from core 1351to substantially avoid (within operationally acceptable limits) opticalcoupling therewith, and to similarly substantially avoid opticalcoupling with waveguide core 1310 upon assembly (over a range ofrelative waveguide positions within the assembly tolerance). Separationbetween a waveguide core and an adjacent elongated structural membergreater than about the width of the waveguide core may prove sufficientin may circumstances.

[0038] Various material processing sequences and/or techniques may beemployed for forming substantially flat waveguide and structural uppercladding surfaces. In a first exemplary procedure (FIG. 6), claddingmaterial 620 may be deposited over a portion of the substrate and lowercladding that includes at least a portion of core area 610 andstructural member areas 630 of the core material layer. Claddingmaterial 620 is deposited to the thickness desired for cladding materialabove the area(s) of core material, once the core material layer isdeposited and the desired areas 610 and 630 have been patterned. Theresulting waveguide and structural upper cladding surfaces aresubstantially coplanar, and assembly of two such waveguides results inboth waveguide and structural upper cladding surfaces being positionedagainst one another. Another exemplary procedure (FIG. 7) may beemployed if thicker areas of cladding material are desired elsewhere onthe substrate. A thicker cladding layer 720 may be deposited over thesubstrate, followed by spatially-selective etching of the areaencompassing the structural areas 730 and at least a portion of the corearea 710 of the patterned core material layer. Both the deposition andetch processes tend to preserve the surface topography of the patternedareas of core material. The final thickness of cladding above thepatterned core material (and therefore the height of the waveguide andstructural upper cladding surfaces) is determined in part by timing theetch process, which may or may not be sufficiently precise depending onthe relevant operationally acceptable parameters.

[0039] Another exemplary procedure may be employed (FIG. 8) in whichupper cladding material 820 is first deposited to the thickness desiredfor the waveguide and structural upper cladding. A first etch-stop layerof any suitable type is deposited and patterned so as to cover thestructural core material areas 830 (etch-stop areas 831) and at least aportion of the waveguide core area 810 (etch-stop area 811). Additionalcladding material 820 is then deposited over the substrate to thethickness desired for other areas of the substrate, covering theetch-stop areas 811 and 831. A second etch-stop layer 821 of anysuitable type is deposited and patterned so as to expose the claddingabove the etch-stop layer areas 811 and 831, but protecting areas wherethicker cladding is desired. The entire substrate is then subjected to asuitable etch process, which stops at the respective etch-stop layers.After removal of the (now exposed) etch-stop layers, the desiredwaveguide and structural upper cladding surfaces are ready for assemblywith another waveguide 850 for transverse-coupling. The structuresproduced by the procedures of FIGS. 7 and 8 produce similar waveguidesand structural members, but the procedure of FIG. 8 may offer greaterprecision for achieving a desired upper cladding thickness for thewaveguide core and structural members.

[0040] Each of the foregoing exemplary processes yields substantiallycoplanar waveguide and structural upper cladding surfaces, so that uponassembly for transverse-coupling both waveguide and structural memberupper cladding surfaces are positioned against their counterparts on theother similarly adapted waveguide substrate. This may typically be asuitable arrangement. There may be instances, however, where it isdesirable to assemble waveguides for transverse-transfer of opticalpower therebetween while leaving a gap between the facing waveguideupper cladding surfaces (typically on the order of one or a few tenthsof a micron). The procedures of FIGS. 7 and 8 may each be adapted toyield substantially parallel waveguide and structural upper claddingsurfaces that differ in height above the substrate. In an exemplaryadaptation of the procedure of FIG. 7, an additional etch step may beperformed restricted to an area above core 710, thereby producing awaveguide upper cladding surface lower than the structural uppercladding surfaces. The height difference may be determined throughcontrol of etch parameters employed. Similarly, FIG. 8 may be adapted byemploying an additional etch step restricted to an area above core 810(after selective removal of etch-stop layer 811, for example).

[0041] In the procedure of FIG. 9, after patterning of the core materiallayer to form the core area 910 and structural areas 930, claddingmaterial 920 is deposited over the substrate to the thickness desiredfor the waveguide upper cladding. A suitable etch-stop layer 911 isdeposited and patterned to cover a portion of the core area 910 only.Additional cladding 920 is then deposited over the substrate (coveringthe first etch-stop layer 911) to the thickness desired for thestructural member upper cladding. A second etch-stop layer 931 isdeposited and patterned to cover only the structural member areas 930.Additional cladding 920 is then deposited over the substrate (coveringthe second etch-stop layer 931) to the thickness desired for theremainder of the substrate, and a third etch-stop layer 921 is depositedcovering those areas where thicker cladding is desired (but leavingexposed the desired areas of cladding above the first and secondetch-stop layers). The entire substrate is then subjected to a suitableetch process, which stops at the etch-stop layers over each respectivearea. After removal of the etch-stop layers, the desired waveguide andstructural upper cladding surfaces are ready for assembly with anotherwaveguide 950 for transverse-coupling. The structural upper claddingsurfaces (above areas 930), being higher than the waveguide uppercladding surface, may be positioned against their counterpart areas onanother waveguide substrate while leaving a gap between the facingwaveguide upper cladding surfaces of the assembled transverse-coupledwaveguides.

[0042] It should be noted that the foregoing procedures are exemplary.Many other material processing sequences or procedures may be contrivedto produce waveguide and structural upper cladding surfaces whileremaining within the scope of the present disclosure and/or appendedclaims.

[0043] Each of the foregoing exemplary processes depends at one or morestages on substantially flat cladding material surfaces being formed bydeposition over a low-profile waveguide core. In contrast,chemical-mechanical polishing (CMP) and/or equivalent processingtechnique(s) may be employed to produce substantially flat waveguide andstructural upper cladding surfaces, regardless of the topography of theunderlying core material. While deposition of cladding material over alow-profile core may typically result in waveguide structures resemblingFIGS. 1B and 2B, for example, CMP may be employed to produce thewaveguide structures resembling FIGS. 1E and 2E, for example. CMP and/orequivalent processes may be employed to produce substantially flatsubstantially coplanar waveguide and structural upper cladding surfaces,for a low-profile core area 998 (FIG. 10A) as well as for a core area999 having a height comparable to or even greater than its thickness(FIG. 10B). CMP may be employed to yield separate waveguide andstructural member surfaces similar to those described hereinabove overstructural area 996 and core area 997 of a patterned core material layer(middle step of FIG. 11), or may be carried further to yield a singlesubstantially contiguous substantially flat surface over both waveguidecore and structural areas (last step of FIG. 11). Such a single flatsurface may be assembled with another similarly configured waveguidesubstrate, or may be assembled with a substrate having separatewaveguide and structural upper cladding areas (such as waveguide 1350 ofFIG. 5A, for example).

[0044] Transparent embedding media are frequently employed for securingassembled optical components together and to provide amechanical/moisture/chemical barrier for isolating critical opticalsurfaces from contamination or damage. Such embedding media may fulfillthe function of more traditional hermetic packaging, and may frequentlytake the form of polymer precursors that are applied to an opticalassembly in liquid form, allowed to flow into and fill the desiredvolumes in and around the optical assembly, and then cured to form asubstantially solid embedding medium surrounding the assembled opticalcomponents. Such embedding may also serve to reduce the index contrastbetween various components of the optical assembly and the surroundingenvironment. Reduced index contrast may serve to: reduce unwantedreflections at transmissive component surfaces; reduce opticalscattering and/or unwanted optical coupling due to imperfect orirregular component surfaces; reduce diffractive losses for an opticalwaveguide end-coupled with another optical waveguide, component, ordevice; loosen translational and/or angular alignment tolerances fortransverse-coupled or end-coupled optical components; and/or reduceoptical losses and/or unwanted optical coupling due to mechanicaljuxtaposition of transverse-coupled optical components. An exemplaryoptical assembly is shown in FIGS. 12A-12B including transverse-coupledoptical waveguides 1600 (including core 1610, cladding 1620, and supportmembers 1630 on substrate 1602) and 1700 (including core 1710, cladding1720, and support members 1730 on substrate 1702; waveguide 1700integrated and end-coupled with optical device 1704). Waveguide 1700terminates at end face 1701, while waveguide 1600 terminates at end face1601. Along the waveguide segments where optical transverse-couplingoccurs, cores 1610 and 1710 are each provided with relatively thin uppercladding (cladding 1620 and 1720, respectively; typically less than 1 μmthick). The thin upper cladding makes the respective cores accessiblefor transverse-coupling. Unacceptable levels of optical loss and/orundesirable optical mode coupling may be induced: i) in waveguide 1700by device end face 1703; ii) in waveguide 1700 by the abrupt appearanceof waveguide 1600 at end face 1601; iii) in waveguide 1600 by the abruptappearance of waveguide 1700 at end face 1701; iv) in waveguide 1600 bythe abrupt appearance of a thicker upper cladding layer at face 1603.Surface irregularities and/or contamination along the sides and/orexposed surfaces of waveguides 1600 and/or 1700 may also lead tounacceptable levels of optical loss and/or undesirable optical modecoupling. Filling spaces 1801, 1802, 1803, and 1804 with an embeddingmedium having an index near that of cladding 1620 and/or 1720 (or atleast nearer than an index of unity) serves to reduce such opticallosses and mode couplings. If the embedding material index substantiallymatches that of cladding 1620 and 1720, losses and mode coupling fromthese sources may be substantially eliminated.

[0045] To have the desired effect, an embedding medium must coversubstantially uniformly the relevant optical surfaces. If the coverageis non-uniform, optical losses and/or undesirable optical mode couplingsmay not be sufficiently reduced, and may even be increased relative to anon-embedded optical assembly. Substantially uniformly filling volumes1803 and 1804 may prove problematic, due to the elongated shape andrelatively thin vertical extent (less than 0.5 μm for silicon nitridecores 1610 and 1710, for example). Surface tension and/or viscosity ofthe embedding precursor, as well as air trapped within these volumes,may not always result in uniform filling of volumes 1803/1804. Supportstructures 1630 and/or 1730 may be segmented (as in FIG. 13), leavinglateral channels 1805 between the support structure segments. Theselateral channels provide multiple flow paths (indicated by arrows) forembedding precursor to substantially fill all of the required volumes1803 and 1804, while also providing a path for air to escape as theembedding material flows in. Flow channels of differing depths may beemployed for controlling the flow of embedding material. For example, adeeper longitudinal channel 1806 may provide rapid flow, so that slowerflow through lateral channels 1805 flows in the same direction for allsuch channels. Such unidirectional flow may result in more uniformfilling of volumes 1803/1804. Support structures on both of theassembled waveguide substrates may be segmented in this way, or on onlyone or the other of the assembled waveguide substrates. It may bedesirable to provide additional structures (not shown) similar tosupport structures 1630/1730 on one or both of substrates 1602 and 1702near the respective waveguides, not necessarily to provide additionalmechanical support, but to further guide the flow of embedding precursoron the optical assembly (directing flow in some instances, diverting orpreventing flow in other instances).

[0046] Additional structures may be employed elsewhere on a waveguidesubstrate for guiding the flow of embedding material precursor prior tocuring. FIG. 14 shows a waveguide substrate 1902 with waveguides 1900and optical devices 1910 assembled onto substrate 1902. Concealedbeneath devices 1910 are optical transverse-coupled waveguides andsupport structures as shown in FIGS. 12A-12B and FIG. 13 (includingsegmented support structures for facilitating embedding precursor flowaround the waveguides). Also shown is a gutter 1920 formed around theoptical assembly, for limiting the extent of the flow of the embeddingprecursor. Excess precursor would flow into the gutter, either to remainthere or to flow off of the substrate wafer, through saw cut 1922 inthis example. Structures 1930, similar in form to support structures1630/1640 of FIGS. 12A-12B and FIG. 13, are shown limiting precursorflow near one of devices 1910. Such gutters and barriers may beconfigured in a variety of ways for meeting particular structuralrequirements for the location of embedding material, while remainingwithin the scope of the present disclosure and/or appended claims.

[0047] It should be noted that such embedding flow control structures,as with other structures formed on substrate 1902, may be formed usingspatially selective material processing on a wafer scale concurrentlyfor many substrates 1902. Assembly of components 1910 onto substrate1902 may be performed on a wafer scale for multiple substrates 1902prior to division of the wafer, or such assembly may be performed afterdivision of the wafer (either at the “bar” level for multiple substrates1902 in single rows divided from the wafer, or at the individualsubstrate level). If assembly is performed prior to division of thewafer or at the bar level, then application of embedding precursor tosubstrate 1902 and assembled components 1902 may also be performed priorto division of the substrate wafer or at the bar level, respectively.

[0048] A variety of optical waveguides, optical devices, and/or opticalcomponents may be secured and embedded on a planar waveguide substrateas described hereinabove. Embedded optical components and/or waveguidesmay be transverse-coupled, end-coupled, or otherwise arranged forachieving the desired optical functionality. Embedding of suchcomponents and/or waveguides shall fall within the scope of the presentdisclosure and/or appended claims.

[0049] There are many suitable materials that may be employed forembedding optical waveguides and other optical components and/or deviceson a planar waveguide substrate. Silicone and silicone-based polymer ofvarious sorts have been successfully employed for such embedding. Othersuitable materials may include but are not limited to polyimides,epoxies, CYTOP (Asahi Glass Company; a poly-fluorinated polymericmaterial that may be cross-linked), silicone and silicone-basedpolymers, siloxane polymers, Cyclotene™ (B-staged bis-benzocyclobutene,Dow), Teflon® AF (DuPont), or other polymers. Various of these materialsmay have significantly temperature-dependent refractive indices. If suchmaterials are employed, this temperature dependency must be compensatedin some instances, may be exploited in other instances for active devicecontrol, or may be safely ignored in still other instances.

[0050] For purposes of the foregoing written description and/or theappended claims, “index” may denote the bulk refractive index of aparticular material (also referred to herein as a “material index”) ormay denote an “effective index” n_(eff), related to the propagationconstant β of a particular optical mode in a particular optical elementby β=2πn_(eff)/λ. The effective index may also be referred to herein asa “modal index”. As referred to herein, the term “low-index” shalldenote any materials and/or optical structures having an index less thanabout 2.5, while “high-index” shall denote any materials and/orstructures having an index greater than about 2.5. Within these bounds,“low-index” may refer to: silica (SiO_(x)), germano-silicate,boro-silicate, other doped silicas, and/or other silica-based materials;silicon nitride (Si_(x)N_(y)) and/or silicon oxynitrides (SiO_(x)N_(y));other glasses; other oxides; various polymers; and/or any other suitableoptical materials having indices below about 2.5. “Low-index” may alsoinclude optical fiber, optical waveguides, planar optical waveguides,and/or any other optical components incorporating such materials and/orexhibiting a modal index below about 2.5. Similarly, “high-index” mayrefer to materials such as semiconductors, IR materials, and/or anyother suitable optical materials having indices greater than about 2.5,and/or optical waveguides of any suitable type incorporating suchmaterial and/or exhibiting a modal index greater than about 2.5. Theterms “low-index” and “high-index” are to be distinguished from theterms “lower-index” and “higher-index”, also employed herein.“Low-index” and “high-index” refer to an absolute numerical value of theindex (greater than or less than about 2.5), while “lower-index” and“higher-index” are relative terms indicating which of two particularmaterials has the larger index, regardless of the absolute numericalvalues of the indices.

[0051] For purposes of the foregoing written description and/or theappended claims, the term “optical waveguide” (or equivalently,“waveguide”) as employed herein shall denote a structure adapted forsupporting one or more optical modes. Such waveguides shall typicallyprovide confinement of a supported optical mode in two transversedimensions while allowing propagation along a longitudinal dimension.The transverse and longitudinal dimensions/directions shall be definedlocally for a curved waveguide; the absolute orientations of thetransverse and longitudinal dimensions may therefore vary along thelength of a curvilinear waveguide, for example. Examples of opticalwaveguides may include, without being limited to, various types ofoptical fiber and various types of planar waveguides. The term “planaroptical waveguide” (or equivalently, “planar waveguide”) as employedherein shall denote any optical waveguide that is provided on asubstantially planar substrate. The longitudinal dimension (i.e., thepropagation dimension) shall be considered substantially parallel to thesubstrate. A transverse dimension substantially parallel to thesubstrate may be referred to as a lateral or horizontal dimension, whilea transverse dimension substantially perpendicular to the substrate maybe referred to as a vertical dimension. Examples of such waveguidesinclude ridge waveguides, buried waveguides, semiconductor waveguides,other high-index waveguides (“high-index” being above about 2.5),silica-based waveguides, polymer waveguides, other low-index waveguides(“low-index” being below about 2.5), core/clad type waveguides,multi-layer reflector (MLR) waveguides, metal-clad waveguides,air-guided waveguides, vacuum-guided waveguides, photonic crystal-basedor photonic bandgap-based waveguides, waveguides incorporatingelectro-optic (EO) and/or electro-absorptive (EA) materials, waveguidesincorporating non-linear-optical (NLO) materials, and myriad otherexamples not explicitly set forth herein which may nevertheless fallwithin the scope of the present disclosure and/or appended claims. Manysuitable substrate materials may be employed, including semiconductor,crystalline, silica or silica-based, other glasses, ceramic, metal, andmyriad other examples not explicitly set forth herein which maynevertheless fall within the scope of the present disclosure and/orappended claims.

[0052] One exemplary type of planar optical waveguide that may besuitable for use with optical components disclosed herein is a so-calledPLC waveguide (Planar Lightwave Circuit). Such waveguides typicallycomprise silica or silica-based waveguides (often ridge or buriedwaveguides; other waveguide configuration may also be employed)supported on a substantially planar silicon substrate (typically with aninterposed silica or silica-based optical buffer layer). Sets of one ormore such waveguides may be referred to as planar waveguide circuits,optical integrated circuits, or opto-electronic integrated circuits. APLC substrate with one or more PLC waveguides may be readily adapted formounting one or more optical sources, lasers, modulators, and/or otheroptical devices adapted for end-transfer of optical power with asuitably adapted PLC waveguide. A PLC substrate with one or more PLCwaveguides may be readily adapted (according to the teachings ofearlier-cited U.S. application Ser. No. 60/334,705, U.S. applicationSer. No. 60/360,261, U.S. application Ser. No. 10/187,030, and/or U.S.application Ser. No. 60/466,799) for mounting one or more opticalsources, lasers, modulators, and/or other optical devices adapted fortransverse-transfer of optical power with a suitably adapted PLCwaveguide (mode-interference-coupled, or substantially adiabatic,transverse-transfer; also referred to as transverse-coupling).

[0053] For purposes of the foregoing written description and/or appendedclaims, “spatially-selective material processing techniques” shallencompass epitaxy, layer growth, lithography, photolithography,evaporative deposition, sputtering, vapor deposition, chemical vapordeposition, beam deposition, beam-assisted deposition, ion beamdeposition, ion-beam-assisted deposition, plasma-assisted deposition,wet etching, dry etching, ion etching (including reactive ion etching),ion milling, laser machining, spin deposition, spray-on deposition,electrochemical plating or deposition, electroless plating,photo-resists, UV curing and/or densification, micro-machining usingprecision saws and/or other mechanical cutting/shaping tools, selectivemetallization and/or solder deposition, chemical-mechanical polishingfor planarizing, any other suitable spatially-selective materialprocessing techniques, combinations thereof, and/or functionalequivalents thereof. In particular, it should be noted that any stepinvolving “spatially-selectively providing” a layer or structure mayinvolve either or both of: spatially-selective deposition and/or growth,or substantially uniform deposition and/or growth (over a given area)followed by spatially-selective removal. Any spatially-selectivedeposition, removal, or other process may be a so-called direct-writeprocess, or may be a masked process. It should be noted that any “layer”referred to herein may comprise a substantially homogeneous materiallayer, or may comprise an inhomogeneous set of one or more materialsub-layers. Spatially-selective material processing techniques may beimplemented on a wafer scale for simultaneous fabrication/processing ofmultiple structures on a common substrate wafer.

[0054] It should be noted that various components, elements, structures,and/or layers described herein as “secured to”, “connected to”,“deposited on”, “formed on”, or “positioned on” a substrate may makedirect contact with the substrate material, or may make contact with oneor more layer(s) and/or other intermediate structure(s) already presenton the substrate, and may therefore be indirectly “secured to”, etc, thesubstrate.

[0055] The phrase “operationally acceptable” appears herein describinglevels of various performance parameters of optical components and/oroptical devices, such as optical power transfer efficiency(equivalently, optical coupling efficiency), optical loss, undesirableoptical mode coupling, and so on. An operationally acceptable level maybe determined by any relevant set or subset of applicable constraintsand/or requirements arising from the performance, fabrication, deviceyield, assembly, testing, availability, cost, supply, demand, and/orother factors surrounding the manufacture, deployment, and/or use of aparticular optical device. Such “operationally acceptable” levels ofsuch parameters may therefor vary within a given class of devicesdepending on such constraints and/or requirements. For example, a loweroptical coupling efficiency may be an acceptable trade-off for achievinglower device fabrication costs in some instances, while higher opticalcoupling may be required in other instances in spite of higherfabrication costs. The “operationally acceptable” coupling efficiencytherefore varies between the instances. In another example, higheroptical loss (due to scattering, absorption, undesirable opticalcoupling, and so on) may be an acceptable trade-off for achieving lowerdevice fabrication cost or smaller device size in some instances, whilelower optical loss may be required in other instances in spite of higherfabrication costs and/or larger device size. The “operationallyacceptable” level of optical loss therefore varies between theinstances. Many other examples of such trade-offs may be imagined.Optical devices and fabrication methods therefor as disclosed herein,and equivalents thereof, may therefore be implemented within tolerancesof varying precision depending on such “operationally acceptable”constraints and/or requirements. Phrases such as “substantiallyadiabatic”, “substantially spatial-mode-matched”, “substantiallymodal-index-matched”, “so as to substantially avoid undesirable opticalcoupling”, and so on as used herein shall be construed in light of thisnotion of “operationally acceptable” performance.

[0056] While particular examples have been disclosed herein employingspecific materials and/or material combinations and having particulardimensions and configurations, it should be understood that manymaterials and/or material combinations may be employed in any of avariety of dimensions and/or configurations while remaining within thescope of inventive concepts disclosed and/or claimed herein.

[0057] It is intended that equivalents of the disclosed exemplaryembodiments and methods shall fall within the scope of the presentdisclosure and/or appended claims. It is intended that the disclosedexemplary embodiments and methods, and equivalents thereof, may bemodified while remaining within the scope of the present disclosureand/or appended claims.

What is claimed is:
 1. An optical apparatus, comprising: a first planaroptical waveguide comprising a first waveguide core within a firstcladding, an upper surface of the first cladding over the first corebeing substantially flat along at least a portion of the length thereof,thereby forming a first substantially flat waveguide upper claddingsurface; and a second planar optical waveguide comprising a secondwaveguide core within a second cladding, an upper surface of the secondcladding over the second core being substantially flat along at least aportion of the length thereof, thereby forming a second substantiallyflat waveguide upper cladding surface, the first and second planaroptical waveguides assembled together with at least portions of theircorresponding substantially flat waveguide upper cladding surfacespositioned facing one another, thereby positioning the first and secondplanar optical waveguides for optical transverse-coupling between thefirst and second cores along corresponding transverse-coupled portionsthereof.
 2. The apparatus of claim 1, wherein the first and secondplanar optical waveguides are assembled together with theircorresponding substantially flat waveguide upper cladding surfacespositioned against one another.
 3. The apparatus of claim 1, wherein thefirst and second planar optical waveguides are assembled together withtheir corresponding substantially flat waveguide upper cladding surfacesspaced-apart from one another.
 4. The apparatus of claim 1, wherein atleast one of the first and second waveguide cores has a lateraldimension thereof that is larger than a vertical dimension thereof alonga portion of the waveguide core below the corresponding substantiallyflat waveguide upper cladding surface.
 5. The apparatus of claim 1,further comprising: at least one additional area of first core materialwithin the first cladding, the additional area of first core materialforming a corresponding substantially flat first structural uppercladding surface substantially parallel to the first substantially flatwaveguide upper cladding surface; and at least one additional area ofsecond core material within the second cladding, the additional area ofsecond core material forming a corresponding substantially flat secondstructural upper cladding surface substantially parallel to the secondsubstantially flat waveguide upper cladding surface, the first andsecond structural upper cladding surfaces being positioned against oneanother upon assembly of the first and second planar waveguides with thecorresponding waveguide upper cladding surfaces facing one another. 6.The apparatus of claim 5, wherein the first waveguide upper claddingsurface and the first structural upper cladding surface arenon-coplanar, thereby positioning, upon assembly of the first and secondplanar waveguides, the first and second waveguides with theircorresponding substantially flat upper waveguide cladding surfacesspaced-apart from one another.
 7. The apparatus of claim 5, wherein thefirst waveguide upper cladding surface and the first structural uppercladding surface are substantially coplanar.
 8. The apparatus of claim5, wherein the first waveguide upper cladding surface and the firststructural upper cladding surface are substantially coplanar; the secondwaveguide upper cladding surface and the second structural uppercladding surface are substantially coplanar; and the first and secondwaveguide upper cladding surfaces are positioned against one anotherupon assembly of the first and second planar waveguides with the firstand second structural upper cladding surfaces positioned against oneanother.
 9. The apparatus of claim 1, further comprising: a pair ofadditional areas of first core material disposed within the firstcladding on opposite sides of the first waveguide core, each of the pairof additional areas of first core material comprising an elongated arearunning substantially parallel to and laterally spaced apart from thefirst waveguide core, the pair of additional areas of first corematerial forming a corresponding first pair of structural upper claddingsurfaces substantially parallel to the first substantially flatwaveguide upper cladding surface; and a pair of additional areas ofsecond core material disposed within the second cladding on oppositesides of the second waveguide core, each of the pair of additional areasof second core material comprising an elongated area runningsubstantially parallel to and laterally spaced apart from the secondwaveguide core, the pair of additional areas of second core materialforming a corresponding second pair of structural upper claddingsurfaces substantially parallel to the second substantially flatwaveguide upper cladding surface, the first and second pairs ofstructural upper cladding surfaces being positioned against one anotherupon assembly of the first and second planar waveguides with thecorresponding waveguide upper cladding surfaces facing one another. 10.The apparatus of claim 9, wherein the first waveguide upper claddingsurface and the first pair of structural upper cladding surfaces arenon-coplanar, thereby positioning, upon assembly of the first and secondplanar waveguides, the first and second waveguides with theircorresponding substantially flat upper waveguide cladding surfacesspaced-apart from one another.
 11. The apparatus of claim 9, wherein thefirst waveguide upper cladding surface and the first pair of structuralupper cladding surfaces are substantially coplanar.
 12. The apparatus ofclaim 9, wherein the first waveguide upper cladding surface and thefirst pair of structural upper cladding surfaces are substantiallycoplanar; the second waveguide upper cladding surface and the secondpair of structural upper cladding surfaces are substantially coplanar;the first and second waveguide upper cladding surfaces are positionedagainst one another upon assembly of the first and second planarwaveguides with the first and second pairs of structural upper claddingsurfaces positioned against one another.
 13. The apparatus of claim 9,wherein the pair of additional areas of first core material arelaterally spaced apart from the first waveguide core by a distance atleast as large as the width of the first waveguide core; and the pair ofadditional areas of second core material are laterally spaced apart fromthe second waveguide core by a distance at least as large as the widthof the second waveguide core.
 14. The apparatus of claim 9, wherein thepair of additional areas of first core material are laterally spacedapart from the first waveguide core by a distance sufficiently large soas to substantially avoid optical coupling between the pair ofadditional areas of first core material and each of the first and secondwaveguide cores; and the pair of additional areas of second corematerial are laterally spaced apart from the second waveguide core by adistance sufficiently large so as to substantially avoid opticalcoupling between the pair of additional areas of second core materialand each of the first and second waveguide cores.
 15. The apparatus ofclaim 9, further comprising embedding material substantially filling avolume between the respective upper cladding surfaces of the assembledfirst and second waveguides, the volume disposed between the engagedpairs of substantially flat structural upper cladding surfaces of theassembled waveguides.
 16. The apparatus of claim 9, at least oneelongated area of core material having therethrough at least one gap,the gap providing a flow channel for a liquid precursor for an embeddingmedium to flow into and substantially fill a volume between therespective upper cladding surfaces of the assembled first and secondwaveguides, the volume disposed laterally between the engaged pairs ofsubstantially flat structural upper cladding surfaces.
 17. The apparatusof claim 1, wherein index contrast between at least one of the first andsecond cores and the corresponding cladding is less than about 5%. 18.The apparatus of claim 17, wherein at least one of the first and secondcores comprises doped silica and the corresponding cladding comprisessilica or doped silica.
 19. The apparatus of claim 17, wherein at leastone of the first and second cores is less than about 1.5 μm in avertical dimension and less than about 6 μm in a lateral dimension. 20.The apparatus of claim 1, wherein index contrast between at least one ofthe first and second cores and the corresponding cladding is greaterthan about 5%.
 21. The apparatus of claim 20, wherein at least one ofthe first and second cores comprises silicon nitride or siliconoxynitride and the corresponding cladding material comprises silica ordoped silica.
 22. The apparatus of claim 20, wherein at least one of thefirst and second cores is less than about 200 nm in a vertical dimensionand less than about 5 μm in a lateral dimension.
 23. The apparatus ofclaim 1, wherein at least one of the first and second cladding is lessthan about 1 μm thick over the transverse-coupled portion of thecorresponding core.
 24. The apparatus of claim 1, wherein at least oneof the first and second cladding is less than about 0.5 μm thick overthe transverse-coupled portion of the corresponding core.
 25. Theapparatus of claim 1, further comprising at least one additional area ofcore material within at least one of the first cladding and the secondcladding, with a corresponding area of upper cladding surface, thecorresponding area of the upper cladding surface forming a flow-directorfor an embedding medium applied to at least one of the first and secondplanar waveguides.
 26. The apparatus of claim 1, further comprisingembedding material substantially filling a volume between respectiveupper cladding surfaces of the assembled first and second waveguides.27. The apparatus of claim 1, wherein at least one of the first andsecond cores terminates at at least one end thereof, the terminatingwaveguide core tapering in the lateral dimension along thetransverse-coupled portion thereof toward the terminated end.